Scientists Puzzle Over Hurricane Ike's "Forerunner" Wave

Rising water began to block roads near
Galveston, Texas, far in advance of Hurricane Ike's landfall and
official storm surge. Photo courtesy Jocelyn
Augustino/FEMA
More than a year after Hurricane Ike made landfall near
Galveston, Texas, some scientists are still puzzling over one of
the storm's unusual traits — rising water that flooded some
parts of the Texas coast far in advance of the storm's anticipated
surge. "There's something called a forerunner that isn't showing up
in any of the computer models," North Carolina Sea Grant storm
flooding expert
Spencer Rogers told Coastal Connection in November.
The forerunner wave appears to have caused Ike's high water to
arrive earlier, stay longer, and cause more damage than was
otherwise expected. The early flooding may also have contributed to
Ike's tragic fatalities. That's because some residents who had
planned to evacuate low-lying areas ahead of the storm found their
escape routes blocked much earlier than they had anticipated.
Unable to make their way to safety, some of the locals became
victims of the storm: trapped, swept away, and drowned when the
main surge and its punishing waves did arrive.
"It was actually flooded on the Bolivar Peninsula for over 20
hours," says Rogers. "The flooding came in way early. Normally,
four or five hours would be a long period for peak flooding. But
that early wave flooded everything, so people couldn't get off the
island -- 24 hours in advance, practically."

A few days after the storm, USA Today told the story of Texas
builder Bobby Anderson, who was trapped with a woman friend in
Gilchrist on the Bolivar Peninsula, when the two found their way
blocked by four feet of water on the highway in early afternoon
("Man
stricken with loss when rescue never came," by Allen G. Breed,
AP National Writer). The pair took refuge in the upper floors of a
partially built house — but Anderson's companion lost her
grip on the house's rafters during the night and was washed
away.
Officially, Hurricane Ike made landfall as just a Category 2
hurricane on the Saffir-Simpson scale — a fact that deceived
some coastal residents into underestimating the force and fury of
the storm's accompanying surge. But weather scientists were aware
well in advance that Ike's surge was sure to exceed the typical
wave brought by a Category 2 storm. Ike's peak winds were only part
of the equation, they pointed out: the very wide area covered by
the storm meant that the energy in the water wave it created was
likely to be devastating. Expert Jeff Masters, PhD., for instance,
warned on his
Weather Underground blog, "The amount of water Ike has put in
motion is about 10% greater than what Katrina did, and thus we can
expect Ike's storm surge damage will be similar to or greater than
Katrina's." And official government warnings minced no words: "ALL
NEIGHBORHOODS...AND POSSIBLY ENTIRE COASTAL COMMUNITIES... WILL BE
INUNDATED DURING THE PERIOD OF PEAK STORM TIDE. PERSONS NOT HEEDING
EVACUATION ORDERS IN SINGLE FAMILY ONE OR TWO STORY HOMES WILL FACE
CERTAIN DEATH." This YouTube archive of Associated Press coverage
before Ike's arrival shows Texas officials telling residents,
"Get off the
roads now".
It was all very clear and direct — except that neither Dr.
Masters' scholarly discussion nor the Weather Service warnings made
mention of any early-arriving wave. In fact, scientists did not
expect the early-rising water — and it took months after the
storm before they were sure that the "forerunner wave" had happened
at all.
Historically, says Spencer Rogers, researchers have not had good
information on storm surges. "We never had any data," he explains.
"In storm surges, all the tide gauges fail." But starting with
Hurricane Rita, he says, U.S. Geological Survey scientists have
been placing special metering devices in the path of oncoming
storms. "These are simple, small pressure gauges that they tie to
something that they think is going to be there after the storm," he
says. "And it's like the wind towers people set up in front of the
storms — sooner or later you are gonna get lucky, and we did
get lucky for Ike. There were a few gauges that survived Ike, and
right in the middle of our study area, we know what the water level
was. I can tell you that it was 20 hours, because we have a gauge
recording of it. And that, we have never had in earlier storms. So
now that we can see it, and prove what happened, we can go back in
and try and figure out how we would identify it in advance, and
maybe do some predictions. But putting it into a storm surge model
is much harder and more complicated. That's a lot more years down
the line."
Even the cause of the wave is a mystery, says Rogers. "One
theory is that it's a wave generated internally within the gulf,
that is rotating around the Gulf of Mexico, driven by a
Gulf-of-Mexico-scale hurricane effect. The hurricane coming into
the gulf would initiate it [watch this satellite time-lapse video
of Ike moving into the Gulf and across to Texas]. But for that
rotating wave to be a problem, that has to hit at the target area
at the same time that the storm does. So if they're out of sync, it
may not be a factor."

Until meteorologists and oceanographers get a better handle on
the dynamics of the phenomenon, anticipating hurricane flooding and
wave effects will remain more of an art than a science. In South
Louisiana, Ike brought not just an early surge, but also a "second
surge," reports the Baton Rouge Advocate ("Second
surge hidden hazard of hurricanes," by Amy Wold). There, Port
Fourchon Harbor Police Chief Jon Callais told the Advocate, crews
planning to evacuate municipal trucks were surprised by two feet of
floodwater on the highway more than 36 hours before Ike made
landfall — and the hurricane did not even pass over the area.
Other residents evacuated, then returned when water receded —
only to have to leave a second time when rising water returned.
The forerunner effect is an exception, notes Spencer Rogers, and
may not even occur with Atlantic Coast hurricanes. "It's in no way
typical," Rogers says. "Where I live in North Carolina, I was in
the eye of four hurricanes between 1996 and 1999, and then in the
radius of maximum winds for a fifth one. We were close enough to
the target areas that we got good measurements of storm surge rise
and fall, in my backyard. And none of those storms had a
forerunner. This is not something that occurs every single
time."
But for Gulf Coast residents, the forerunner threat is a good
reason to heed evacuation warnings — and to heed them early.
And it's yet one more reason why in coming decades, low-lying areas
near the Gulf are likely to face increasing pressure from flood and
erosion, as well as from storm winds.